1. Field of the Invention
The invention relates to fuel assemblies for nuclear reactors and it is particularly suitable for use in light water cooled and moderated reactors.
2. Description of the Prior Art
Most presently used nuclear fuel assemblies include a bundle of fuel rods (this term designating not only rods loaded with fissile material but also rods loaded with a fertile material) and a structure for holding the bundle, comprising upper and lower end pieces or nozzle connected together by tie rods (frequently serving as guide tubes for control rods) which carry grids for holding the fuel rods at the nodes of a regular lattice, and resilient means for transmitting forces to the upper end piece, forcibly applying the fuel assembly on a core support plate and having a hold down function.
A nuclear reactor core typically consists of such assemblies, of hexagonal or square cross-section, carried by a core support plate. The elements of the structure are fixed rigidly to each other. The end pieces have passages for the coolant and the core support plate has openings for coolant flow into the assemblies. The support plate has centering studs for engagement in the lower end piece for indexing it.
An upper core plate over the assemblies defines the core and has openings for discharging the coolant after it has been heated in the core. This upper plate constitutes, with elements for guiding control rod clusters, upper internal equipments of the reactor.
Due to the hydraulic thrust of the pressurized coolant on the fuel assemblies under normal operation and/or under transitory operating conditions, the assemblies must be provided with hold down devices for retaining them in contact with the core support plate. Such devices generally use resilient means. The most widely used approach consists in using springs fixed to the upper end piece of the assembly, on which the upper core plate bears so as to exert a force applying the assembly on the core support plate against the hydraulic thrust of the coolant. These springs may take on very different shapes, for example those described in French Nos. 1 536 257, 2 326 010, 2 412 142 and 2 409 576 to which reference may be made.
These spring devices operate substantially satisfactorily. They nevertheless raise problems. They are complex to manufacture. Their efficiency decreases progressively during irradiation. The space required for them decreases the coolant flow area in the upper end piece and increases the head loss. The springs hinder the introduction, into the upper end piece, of the tool for handling the assembly. The problems, or at least some of them, increase in seriousness on assemblies of great length. A major problem is created by the growth differential under irradiation and at high temperature, between the different constituents of the core and the assemblies, due to different materials being used (as zirconium base alloy and stainless steel). In present assemblies, having a length of several meters, the difference in length between adjacent assemblies, one of which is new and another has undergone irradiation, may be as high as several centimeters. The differences in length make it difficult to provide springs capable of permanently applying under all circumstances the assemblies against the core support plate, while this is necessary to avoid shocks and excessive vibrations of the rods. In addition, the steady increase in the power of nuclear reactors results in an increase of the hydraulic forces on the springs, which forces may become greater than the weight of the assembly, and require the use of larger and larger springs more and more difficult to integrate into the upper end piece.